The present invention relates to a solid-state image pickup device constructed with charged coupled devices (CCDs) of the interline transfer type, and more particularly to a solid-state image pickup device improved in that the number of picture elements is increased and its vignetting factor is increased by applying the vertical charge transfer paths within a light-receiving area to photosensitive portions (picture elements).
A conventional solid-state image pickup device will be described with reference to FIG. 6. In FIG. 6, reference numeral 1 designates a light-receiving area; 2, a horizontal charge transfer path; and 3, an output amplifier. Photodiodes are vertically and horizontally arranged into an n.times.m matrix array within the light-receiving area 1. Color filters of green (G) are layered on the light incident surfaces of the photodiodes linearly arrayed in even-numbered rows, thereby to form pixels with green spectral sensitivity a.sub.2,1, a.sub.2,2. . . , a.sub.2,m, a.sub.4,1, a.sub.4,2, . . . , a.sub.4,m, a.sub.n,1, a.sub.n,2, . . . , a.sub.n,m.
Vertical charge transfer paths l.sub.1, l.sub.2, . . . , l.sub.m and groups of blue and green pixels linearly arrayed in columns are alternately arranged. Transfer electrodes (not shown) made of polysilicon, which are used for transferring signal charge in the vertical direction in response to drive signals based on the so-called four-phase drive system, are layered on respective ones of the vertical charge transfer paths.
The terminals of the vertical charge transfer paths l.sub.1, l.sub.2, . . . , l.sub.m are coupled with the horizontal charge transfer path 2, while being disposed side by side. The output amplifier 3 is provided at the output terminal of the horizontal charge transfer path 2.
A structure of the light-receiving area 1 will be described in detail with reference to FIG. 7.
The structure in the vicinity of the blue pixels a.sub.1,1, a.sub.1,2, and a.sub.1,3 and the green pixels a.sub.2,1, a.sub.2,2, and a.sub.2,3 will be described as a typical example for ease of explanation. In the figure, the blue and green pixels are separated from the vertical charge transfer paths l.sub.1, l.sub.2, and l.sub.3 by channel stoppers as shaded. Pairs of transfer electrodes G.sub.1 and G.sub.2, and G.sub.3 and G.sub.4, each pair being allocated for each linear array of the pixels in the row, are layered on the upper surfaces of the vertical charge transfer paths l.sub.1, l.sub.2, and l.sub.3 (except the upper surfaces of the blue and green pixels). In operation, drive signals .phi..sub.1, .phi..sub.2, .phi..sub.3, and .phi..sub.4 based on the four-phase drive system are applied to the transfer electrodes. In response to the drive signals, potential wells (referred to as transfer elements) for transferring signal charge are generated in the vertical charge transfer paths l.sub. 1, l.sub.2, and l.sub.3.
A transfer gate Tg.sub.1 is inserted between one end of each blue pixel and a transfer element adjacent to the pixel. A transfer gate Tg.sub.2 is inserted between one end of each green pixel and a transfer element adjacent to the pixel. When high voltage signals are applied to the related transfer gates G.sub.2 and G.sub.4, the transfer gates Tg.sub.1 and Tg.sub.2 are rendered conductive.
A sectional structure of the vertical charge transfer paths as taken on line X--X in FIG. 7 will be described with reference to FIG. 8. The transfer electrodes G.sub.1, G.sub.2, G.sub.3 G.sub.4, . . . , which are made of polycrystal, are formed on a silicon oxide film (SiO.sub.2) to serve as a gate oxide film layer, which is formed on the surface of a P-well layer in a semiconductor substrate. In this instance, when an "H" (high) drive signal is applied to the transfer electrodes G.sub.1 and G.sub.2, and an "L" (low) level drive signal is applied to the transfer electrodes G.sub.3 and G.sub.4, a deep potential well is formed under the electrodes G.sub.1 and G.sub.2, and a shallow potential barrier is formed under the transfer electrodes G.sub.3 and G.sub.4. Signal charge can be stored in the well. The potential wells of the transfer elements (as indicated by shading in FIG. 6) are deepened when an image is picked up, and are used as red pixels. For example, in FIG. 7, color filters of red (R) are formed on the upper surfaces of the transfer electrodes G.sub.1, G.sub.3, and G.sub.3. When an image is to be sensed, the drive signals .phi..sub.1 to .phi..sub.3 applied to those electrodes are set to the "H" level, and the drive signal .phi..sub.4 applied to the transfer electrode G.sub.4 is set to the "L" level. The transfer elements by the electrodes G.sub.1 to G.sub.3 are used as red pixels. Those pixels are separated by the potential barrier under the transfer electrode G.sub.4. The pixels of the same color can be formed by applying the drive signals .phi..sub.1 to .phi..sub.4 to the remaining transfer electrodes in the same phase relation.
An image pickup operation of the solid-state image pickup device thus arranged will be described.
During an exposure period, the related drive signals (e.g., .phi..sub.2 to .phi..sub.4) are set to the "H" level, and the remaining drive signal (e.g., .phi.) is set to the "L" level. With such level settings, groups of red pixels are generated in the vertical charge transfer paths. The red pixels thus generated, and the blue and green pixels as constructed with the photodiodes, accumulate signal charge. After the exposure is completed, the red signal charge is transferred toward the horizontal charge transfer path 2 using the four-phase drive system.
In synchronism with the transfer operation, all of the red pixel signals are read out of the horizontal charge transfer path 2. Then, the drive signals whose voltage is higher than a normal voltage are applied to the related transfer electrodes. With voltage is applied, the transfer gates Tg.sub.1 associated with the blue pixels are made conductive and the signal charges of the blue pixels are transferred to the vertical charge transfer path. The transfer gates Tg.sub.1 are then made nonconductive again. Following this, the blue signal charge is transferred toward the horizontal charge transfer path 2 by the four-phase drive system. In synchronism with the transfer operation, all of the blue pixel signals are read out on the horizontal charge transfer path 2. Next, the drive signals whose voltage is higher than a normal voltage are applied to the related transfer electrodes. With application of the high voltage, the transfer gates Tg.sub.2 associated with the green pixels are made conductive, and the signal charges of the green pixels are transferred to the vertical charge transfer path. The transfer gates Tg.sub.2 are then again rendered nonconductive. Following this, the green signal charge is transferred toward the horizontal charge transfer path 2 by the four-phase drive system. In synchronism with the transfer operation, all of the green pixel signals are read out on the horizontal charge transfer path 2.
In this way, the pixel signals are read out every color in an area-sequential scan manner. The pixel signals of these colors thus obtained are equal to one another in number ((n/2).times.m) and resolution, as seen from FIG. 6.
When an image is reproduced by forming luminance signal and color signals from the pixel signals in such image pickup operation, the perceived resolution of the reproduced image becomes better as the high frequency content of the luminance signal is increased. In this respect, there is a strong demand for developing solid-state image pickup devices of high spatial sampling frequencies for the color pixels.